[ui][3D] add EnvironmentMapEntity to display equirectangular images

Load EXR files in the 3D viewport as environment maps mapped on a sphere "attached"  to the camera.
* improve exr loading strategy by considering depthmaps and env maps
* update to DepthMapEntity 2.1 (required to get the loading status)

meshroom/ui/qml/Viewer3D/EnvironmentMapEntity.qml

@@ -0,0 +1,48 @@
+import QtQuick 2.9
+import Qt3D.Core 2.1
+import Qt3D.Render 2.1
+import Qt3D.Extras 2.10
+
+
+/**
+ * EnvironmentMap maps an equirectangular image on a Sphere.
+ * The 'position' property can be used to virually attach it to a camera
+ * and get the impression of an environment at an infinite distance.
+ */
+Entity {
+    id: root
+
+    /// Source of the equirectangular image
+    property url source
+    /// Radius of the sphere
+    property alias radius: sphereMesh.radius
+    /// Number of slices of the sphere
+    property alias slices: sphereMesh.slices
+    /// Number of rings of the sphere
+    property alias rings: sphereMesh.rings
+    /// Position of the sphere
+    property alias position: transform.translation
+
+    components: [
+        SphereMesh {
+            id: sphereMesh
+            radius: 1000
+            slices: 50
+            rings: 50
+        },
+        Transform {
+            id: transform
+            translation: root.position
+        },
+        DiffuseMapMaterial {
+            ambient: "#FFF"
+            shininess: 0
+            specular: "#000"
+            diffuse: TextureLoader {
+                magnificationFilter: Texture.Linear
+                mirrored: true
+                source: root.source
+            }
+        }
+    ]
+}